A liquid crystalline coating liquid containing a lyotropic liquid crystalline compound is used, for example, in producing an optically anisotropic film. Hereafter, the “liquid crystalline coating liquid” may be simply referred to as a “coating liquid”.
Specifically, a lyotropic liquid crystalline compound forms a supramolecular associated body in the coating liquid, and the coating liquid exhibits a liquid crystal phase by this associated body. When a coating film is formed by casting this coating liquid on a base material while applying a shearing force to the coating liquid, the longer axis of the supramolecular associated body is oriented in the casting direction in this coating film. Such a method of forming a coating film by casting a coating liquid is generally referred to as the “solution casting method”. Thereafter, by drying the coating film, a film constituted of the oriented lyotropic liquid crystalline compound is obtained. This film can be used as an optically anisotropic film.
In the meantime, in producing the optically anisotropic film, it is preferable to use a coating liquid that exhibits a stable liquid crystal phase.
Patent Document 1 discloses that a lyotropic liquid crystalline compound may become insoluble in a liquid, and this may become a crystal body to inhibit the exhibition of the liquid crystal phase. In order to prevent this lyotropic liquid crystalline compound from forming a crystal body, Patent Document 1 discloses a method of producing a coating liquid having a step of preparing a pre-treatment liquid containing a solvent and a crystal body of a lyotropic liquid crystalline compound, a step of obtaining an isotropic solution by heat-treating the pre-treatment liquid at a temperature above or equal to the melting point of the crystal body, and a step of cooling the isotropic solution to a temperature below or equal to the temperature at which the liquid crystal phase is exhibited.
According to the aforementioned method disclosed in Patent Document 1, a coating liquid exhibiting a stable liquid crystal phase can be obtained, so that an optically anisotropic film can be produced by forming this into a film by the solution casting method.
However, even in a case in which the lyotropic liquid crystalline compound does not become a crystal body, a fine crystal may be contained in the coating liquid. An optically anisotropic film formed from the coating liquid containing the fine crystal raises a problem of having a large optical scattering and providing a low transparency.    [Patent Document 1]    Japanese Unexamined Patent Publication No. 2009-139806
An object of the present invention is to provide a method of producing a liquid crystalline coating liquid that substantially does not contain a fine crystal.
The present inventors have made eager studies on a cause by which a fine crystal is contained in the coating liquid.
The coating liquid is obtained, for example, by dispersing a lyotropic liquid crystalline compound into a solvent. However, it is practically difficult to use a lyotropic liquid crystalline compound having a purity of 100%. Therefore, a lyotropic liquid crystalline compound that is used contains a little impurity. The present inventors have found out that this impurity is a substance of causing the fine crystal, thereby completing the present invention.
The method of producing a liquid crystalline coating liquid of the present invention contains a heating step of heating a treatment liquid containing a lyotropic liquid crystalline compound containing an impurity, and a solvent, a depositing step of depositing a deposit by cooling the treatment liquid after the heating step to a temperature lower than 40° C., and a removing step of removing the deposit from the treatment liquid.
By the aforementioned method of producing a liquid crystalline coating liquid, most of the impurity contained in the lyotropic liquid crystalline compound is deposited as a deposit, and this is removed from the treatment liquid. For this reason, containment of fine crystals derived from the impurity in the obtained liquid crystalline coating liquid can be suppressed.
In a preferable method of producing a coating liquid of the present invention, the impurity is hardly soluble to the solvent, and the lyotropic liquid crystalline compound is soluble to the solvent.
In another preferable method of producing a coating liquid of the present invention, the deposit is deposited with a fine crystal of the impurity serving as a nucleus in the depositing step.
In another preferable method of producing a coating liquid of the present invention, the aforesaid solvent is an aqueous solvent, and the lyotropic liquid crystalline compound is an aromatic disazo-based compound represented by the following formula (1).

In the general formula (1), Q1 represents a substituted or non-substituted aryl group, Q2 represents a substituted or non-substituted arylene group, R1 independently represents a hydrogen atom, a substituted or non-substituted alkyl group, a substituted or non-substituted acetyl group, a substituted or non-substituted benzoyl group, or a substituted or non-substituted phenyl group, M represents a counter ion, m represents an integer of 0 to 2, n represents an integer of 0 to 6, at least one of m and n is not 0, and 1≦m+n≦6.
The groups OH, (NHR1)m and (SO3M)n shown in the general formula (1) may each be bonded to any one of the seven substitution sites of the naphthyl ring.
In another preferable method of producing a coating liquid of the present invention, the removing step includes a step of filtering the treatment liquid by using a filter having a pore diameter of 1 μm or less.
In another preferable method of producing a coating liquid of the present invention, the removing step includes a step of performing centrifugation on the treatment liquid.
In another aspect of the present invention, an optically anisotropic film is provided.
The optically anisotropic film of the present invention is obtained by applying a coating liquid obtained by the above production method on a development surface to form a coating film and solidifying the coating film.
According to the production method of the present invention, a liquid crystalline coating liquid that substantially does not contain a fine crystal can be obtained. Use of this liquid crystalline coating liquid enables to form an optically anisotropic film excellent in transparency.